The present invention relates to a color recording method for recording images in color by use of three types of translucent photoconductive toner particles in three primary colors, that is, for example, cyan, magenta and yellow, which toner particles are formed in a layer on a doner member, wherein the toner particles are electrically charged to a predetermined polarity, are exposed to optical color images which serve to selectively decrease the resistivities of the toner particles in patterns corresponding to the optical color images, are subjected to selective charge injection into the toner particles with decreased resistivities, resulting in formation of charge patterns in the layer corresponding to the optical images, and are transferred in the image patterns to a recording medium in accordance with the charge patterns.
Conventionally, as a method of recording images in color, a color recording method employing the Carlson electrophotographic process is well known.
The Carlson color recording method is unquestionably an excellent method, except for the twin shortcomings that a relatively long recording time is required, and a large-size apparatus is necessary, in order to separate the colors of an original image into three primary colors and to perform the electrophotographic copying process for reproduction of those primary colors.
Recently, so-called one-shot-full-color recording systems have been developed, which are capable of yielding recorded images in color by a single image exposure. In these recording systems, a photo-electrophoretic method and a particle color separation method are particularly well known. The photo-electrophoretic method is described, for instance, in U.S. Pat. No. 3,553,093 and U.S. Pat. No. 3,383,993, and the particle color separation method is described, for instance, in U.S. Pat. Nos. 4,294,902, 4,284,696, 4,262,078, 4,238,562, 4,230,784 and U.K. Pat. No. 2002913B.
The photo-electrophoretic method utilizes electrophoresis of photoconductive toner particles. More particularly, in the photo-electrophoretic method, three types of photoconductive toner particles in three primary colors are charged to a predetermined polarity, for instance, to a negative polarity, and are dispersed in an electrically insulating liquid medium. A pair of electrodes are placed in this dispersion in such a mode as to subject the toner particles to electrophoresis. One of the pair of electrodes is a transparent electrode charged, for instance, to a positive potential, while the other electrode is negatively charged and has an electrically insulating layer on the surface thereof.
Since the photoconductive toner particles are charged to a negative polarity, they are deposited uniformly on the surface of the transparent positive electrode. When an optical color image is projected on the back side of the transparent electrode, the photoconductive toner particles absorb the light of the optical image and become electrically conductive, with a decrease in the electric resistivities thereof by the light absorption, followed by positive charge injection into those toner particles from the transparent positive electrode, with those toner particles becoming charged to a positive polarity. Since voltage is applied between the two electrodes, the thus positively charged toner particles electrophoretically migrate towards the negative electrode.
As a result of the above-described projection of the optical color image onto the transparent positive electrode, a color image, either in a positive form or a negative form, corresponding to the optical color image, is formed by the toner particles remaining on the transparent positive electrode. The thus formed color image is transferred from the transparent electrode to an image transfer medium, whereby a color image is recorded.
In the particle color separation method, three types of transparent particles in three primary colors, for example, red particles, green particles and blue particles, are uniformly mixed. This mixture is coated in the form of a layer on the surface of a photoconductor. Optical color images are projected onto the photoconductor through the particle layer. Each colored particle contains a sublimational leuco dye which can be colored to the complementary color of the color of that particle. Further, each particle is electrically charged, for example, to a negative polarity and is deposited in the form of a layer on the surface of the photoconductor, which is electrically charged, for example, to a positive polarity.
Under the above-described conditions, optical color images are projected to the particle layer on the photoconductor. When an original contains, for instance, three image areas, a black image area, a white background and a red image area, no light is projected from the black image area onto the layer of the mixed particles, so that the mixture of the particles, that is, red, green and blue, remains in the area corresponding to the black image area on the photoconductor. Thus, the area of the layer of mixed particles on the photoconductor corresponding to the black image area is black in color.
In the area of the layer of mixed particles on the photoconductor corresponding to the white background, light passes through all the particles and reaches the photoconductor. As a result, the portion of the photoconductor where the light of the optical images passes through all the toner particles becomes electrically conductive and electric charges dissipate therefrom, so that the particles in the portion are no longer electrically attracted to the surface of the photoconductor. The particles which are no longer attracted to the photoconductor can be physically removed, for instance, by causing air to blow against the layer of the toner particles, resulting in a plain, i.e., white, background.
In the area of the layer of mixed particles on the photoconductor corresponding to the red image area, the red particles allow the light of the red image to pass therethrough, so that the light which passes through the red particles reaches the photoconductor. As a result, electric charges which attract the red particles to the photoconductor dissipate, and the red particles are no longer attracted to the photoconductor and can be physically removed. The green particles and blue particles remain attracted to the photoconductor.
A sheet of bottom paper for pressure-sensitive copying paper, which is coated, for example, with terra abla, and which serves as a recording sheet, is superimposed on the green and blue particles remaining on the photoconductor and heat is applied to the back side of the bottom paper, so that the leuco dyes contained in those particles are caused to sublime, with formation of a magenta dye from the green particles and yellow dye from the blue particles. The combination of the magenta dye and the yellow dye produces red color. The combination of the magenta dye, yellow dye and cyan dye produces black color in the case of the black image as discussed above.
By the above-mentioned color separation process, followed by the sublimation of the particles to produce their complementary colors, the black image, the plain area and the red image, respectively corresponding to the black image area, the plain background and the red image area of the original, are formed and transferred to the bottom paper which serves as a recording sheet.
With respect to other colors, the above-described color separation mechanism functions similarly.
Thus, the optical color images are subjected to color separation by the three types of transparent particles in the three primary colors contained in the particle layer, and the corresponding color images are recorded on the recording sheet.
The previously described electrophoretic color recording method is theoretically an excellent method. However, it is difficult to embody that method in a commercially acceptable apparatus, since the use of a liquid medium is indispensable.
On the other hand, in the case of the particle color separation method, plain paper cannot be used as the recording medium; rather, it is necessary to use a coated sheet of bottom paper for pressure-sensitive copying paper. Furthermore, as the particle size of the particles employed in that method decreases, appreciable fogging is apt to occur.
Finally, the above-described two methods have the shortcoming that they do not yield high color image density, because there is substantially no overlapping of the photoconductive toner particles and no overlapping of the colored image areas.